The exhaust gas from Diesel engines includes carbon-containing particulates like black smoke (soot), and there is a high demand of reducing the total emission of the carbon-containing particulates, in order to prevent further air pollution. There is a similar demand in direct injection gasoline engines where gasoline is directly injected into a combustion chamber, since the carbon-containing particulates may be discharged with the exhaust gas under some driving conditions.
One proposed technique of remarkably reducing the carbon-containing particulates in the emission from an internal combustion engine disposes a heat-resistant filter in an exhaust conduit of the internal combustion engine and uses the filter to collect the carbon-containing particulates included in the exhaust gas.
The particulates are mainly composed of carbon, but should be exposed to high temperatures of not lower than 550° C. in the exhaust gas containing oxygen for their combustion. As long as the internal combustion engine, such as the Diesel engine or the direct injection gasoline engine, is driven under ordinary conditions, the temperature of the exhaust gas flown into the filter hardly exceeds the temperature of 550° C. It is accordingly required to process the collected particulates by some technique. Otherwise the filter is clogged to cause various troubles, for example, a decrease in output of the internal combustion engine.
A diversity of techniques have been proposed to process the carbon-containing particulates collected by the filter. One simple technique makes a noble metal catalyst, such as platinum, carried on the filter and utilizes the catalytic action for combustion of the collected particulates in the exhaust gas of a relatively low temperature (see JAPANESE PATENT PUBLICATION GAZETTE No. 7-106290). Another proposed technique intentionally raises the temperature of the exhaust gas for combustion of the collected carbon-containing particulates on the filter (see JAPANESE PATENT LAID-OPEN GAZETTE No. 2000-161044). There are a variety of methods applicable to raise the temperature of the exhaust gas. One applicable method called the intake reduction technique sets an on-off valve in an intake conduit of the internal combustion engine and narrows the opening of the valve to increase the temperature of the exhaust gas. Another applicable method delays the injection timing of the fuel from the appropriate timing to increase the temperature of the exhaust gas.
These prior art techniques, however, have some drawbacks discussed below. The catalyst in use naturally deteriorates its performance. The long-term use of the catalyst thus makes it difficult to completely process the collected carbon-containing particulates and eventually leads to clogging of the filter. Increasing the load of the noble metal on the filter certainly prevents the significant deterioration of the catalyst, but it is not desirable to increase the load of the precious noble metal.
The technique of intentionally raising the temperature of the exhaust gas causes the chemical energy of the fuel not to be converted to the output of the internal combustion engine but to be released as heat. This undesirably lowers the output of the internal combustion engine or the fuel consumption rate.
The object of the present invention is thus to solve the drawbacks of the prior art techniques discussed above and to provide a technique of easily and securely regulating and reducing carbon-containing particulates included in the exhaust gas of an internal combustion engine over a long time period without worsening the performances of the internal combustion engine.